Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Oct. 19, 2017	May 15, 2017	Sep. 09, 2016	Dec. 31, 2018	Dec. 31, 2017	Oct. 18, 2017	Jun. 30, 2017	May 14, 2017	Dec. 31, 2016	Sep. 30, 2016	Sep. 08, 2016
Business Acquisition [Line Items]
Goodwill				$ 845,687	$ 842,651
Liabilities Assumed
Goodwill				$ 845,687	842,651
OKSB Merger
Business Acquisition [Line Items]
Cash and due from banks	$ 79,517					$ 79,517
Investment securities	484,173					485,468
Loans acquired	1,996,453				2,021,000	2,039,524
Allowance for loan losses	0					(26,957)
Foreclosed assets	5,157					6,284
Premises and equipment	26,667					21,210
Bank owned life insurance	28,704					28,704
Goodwill	229,144					13,545
Other assets	24,314					33,455
Total assets acquired	2,690,915					2,686,489
Liabilities Assumed
Non-interest bearing transaction accounts	485,971					485,971
Interest bearing transaction accounts and savings deposits	869,252					869,252
Time deposits	611,132					613,345
Total deposits	1,966,355					1,968,568
Securities sold under agreement to repurchase	11,256					11,256
Other borrowings	347,000					347,000
Subordinated debentures	46,393					46,393
Accrued interest and other liabilities	22,804					17,440
Total liabilities assumed	2,393,808					2,390,657
Total equity assumed						295,832
Total liabilities and equity assumed	2,393,808					2,686,489
Net assets acquired	297,107
Purchase price	526,251
Goodwill	229,144					13,545
OKSB Merger | Core deposit intangible
Business Acquisition [Line Items]
Intangibles	42,124					1,933
OKSB Merger | Other intangibles
Business Acquisition [Line Items]
Intangibles	3,806					3,806
OKSB Merger | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks	0
Investment securities	(1,295)
Loans acquired	(43,071)
Allowance for loan losses	26,957
Foreclosed assets	(1,127)
Premises and equipment	5,457
Bank owned life insurance	0
Goodwill	(13,545)
Other assets	(9,141)
Total assets acquired	4,426
Liabilities Assumed
Non-interest bearing transaction accounts	0
Interest bearing transaction accounts and savings deposits	0
Time deposits	(2,213)
Total deposits	(2,213)
Securities sold under agreement to repurchase	0
Other borrowings	0
Subordinated debentures	0
Accrued interest and other liabilities	5,364
Total liabilities assumed	3,151
Total equity assumed	(295,832)
Total liabilities and equity assumed	(292,681)
Goodwill	(13,545)
OKSB Merger | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Intangibles	40,191
OKSB Merger | Fair Value Adjustments | Other intangibles
Business Acquisition [Line Items]
Intangibles	0
First Texas Merger
Business Acquisition [Line Items]
Cash and due from banks	59,277					59,277
Investment securities	80,518					81,114
Loans acquired	2,208,378				$ 2,246,000	2,246,212
Allowance for loan losses	(200)					(20,864)
Premises and equipment	34,987					24,864
Bank owned life insurance	7,190					7,190
Goodwill	240,843					37,227
Other assets	29,748					18,263
Total assets acquired	2,427,226					2,453,283
Liabilities Assumed
Non-interest bearing transaction accounts	74,410					74,410
Interest bearing transaction accounts and savings deposits	1,683,298					1,683,298
Time deposits	123,950					124,233
Total deposits	1,881,658					1,881,941
Securities sold under agreement to repurchase	50,000					50,000
Other borrowings	235,000					235,000
Subordinated debentures	30,912					30,323
Accrued interest and other liabilities	13,396					11,727
Total liabilities assumed	2,210,966					2,208,991
Total equity assumed						244,292
Total liabilities and equity assumed	2,210,966					2,453,283
Net assets acquired	216,260
Purchase price	457,103
Goodwill	240,843					37,227
First Texas Merger | Core deposit intangible
Business Acquisition [Line Items]
Intangibles	7,328					$ 0
First Texas Merger | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks	0
Investment securities	(596)
Loans acquired	(37,834)
Allowance for loan losses	20,664
Premises and equipment	10,123
Bank owned life insurance	0
Goodwill	(37,227)
Other assets	11,485
Total assets acquired	(26,057)
Liabilities Assumed
Non-interest bearing transaction accounts	0
Interest bearing transaction accounts and savings deposits	0
Time deposits	(283)
Total deposits	(283)
Securities sold under agreement to repurchase	0
Other borrowings	0
Subordinated debentures	589
Accrued interest and other liabilities	1,669
Total liabilities assumed	1,975
Total equity assumed	(244,292)
Total liabilities and equity assumed	(242,317)
Goodwill	(37,227)
First Texas Merger | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Intangibles	7,328
Hardeman County Investment Company, Inc.
Business Acquisition [Line Items]
Cash and due from banks		$ 8,001						$ 8,001
Interest bearing balances due from banks - time		1,984						1,984
Investment securities		170,369						170,654
Loans acquired		251,649					$ 254,200	257,641
Allowance for loan losses								(2,382)
Foreclosed assets		631						1,083
Premises and equipment		11,163						9,905
Bank owned life insurance		7,819						7,819
Goodwill		29,371						11,485
Other assets		2,638						2,639
Total assets acquired		462,924						468,829
Liabilities Assumed
Non-interest bearing transaction accounts		76,555						76,555
Interest bearing transaction accounts and savings deposits		214,872						214,872
Time deposits		97,549						97,917
Total deposits		388,976						389,344
Securities sold under agreement to repurchase		17,163						17,163
Other borrowings		3,000						3,000
Subordinated debentures		6,702						6,702
Accrued interest and other liabilities		3,815						1,891
Total liabilities assumed	$ 6,700	419,656						418,100
Total equity assumed								50,729
Total liabilities and equity assumed		419,656						468,829
Net assets acquired		43,268
Purchase price		72,639
Goodwill		29,371						11,485
Hardeman County Investment Company, Inc. | Core deposit intangible
Business Acquisition [Line Items]
Intangibles		7,840						0
Hardeman County Investment Company, Inc. | Other intangibles
Business Acquisition [Line Items]
Intangibles		830						$ 0
Hardeman County Investment Company, Inc. | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks		0
Interest bearing balances due from banks - time		0
Investment securities		(285)
Loans acquired		(5,992)
Allowance for loan losses		2,382
Foreclosed assets		(452)
Premises and equipment		1,258
Bank owned life insurance		0
Goodwill		(11,485)
Other assets		(1)
Total assets acquired		(5,905)
Liabilities Assumed
Non-interest bearing transaction accounts		0
Interest bearing transaction accounts and savings deposits		0
Time deposits		(368)
Total deposits		(368)
Securities sold under agreement to repurchase		0
Other borrowings		0
Subordinated debentures		0
Accrued interest and other liabilities		1,924
Total liabilities assumed		1,556
Total equity assumed		(50,729)
Total liabilities and equity assumed		(49,173)
Goodwill		(11,485)
Hardeman County Investment Company, Inc. | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Intangibles		7,840
Hardeman County Investment Company, Inc. | Fair Value Adjustments | Other intangibles
Business Acquisition [Line Items]
Intangibles		$ 830
Citizens National Bank
Business Acquisition [Line Items]
Cash and due from banks			$ 131,116								$ 131,467
Federal funds sold			10,000								10,000
Investment securities			61,988								61,987
Loans acquired			340,850							$ 348,800	350,361
Allowance for loan losses			0								(4,313)
Foreclosed assets			3,442								4,960
Premises and equipment			8,085								6,746
Bank owned life insurance			6,632								6,632
Goodwill			23,376						$ 21,500
Other assets			17,370								17,364
Total assets acquired			585,149								585,204
Liabilities Assumed
Non-interest bearing transaction accounts			109,281								109,281
Interest bearing transaction accounts and savings deposits			204,912								204,912
Time deposits			195,664								195,664
Total deposits			509,857								509,857
Securities sold under agreement to repurchase			13,233								13,233
FHLB borrowings			4,047								4,000
Accrued interest and other liabilities			5,088								3,558
Total liabilities assumed			532,225								530,648
Total equity assumed			0								54,556
Total liabilities and equity assumed			532,225								585,204
Net assets acquired			52,924
Purchase price			76,300
Goodwill			23,376						$ 21,500
Citizens National Bank | Core deposit intangible
Business Acquisition [Line Items]
Intangibles			5,075								0
Citizens National Bank | Other intangibles
Business Acquisition [Line Items]
Intangibles			591								$ 0
Citizens National Bank | Fair Value Adjustments
Business Acquisition [Line Items]
Cash and due from banks			(351)
Federal funds sold			0
Investment securities			1
Loans acquired			(9,511)
Allowance for loan losses			4,313
Foreclosed assets			(1,518)
Premises and equipment			1,339
Bank owned life insurance			0
Other assets			6
Total assets acquired			(55)
Liabilities Assumed
Non-interest bearing transaction accounts			0
Interest bearing transaction accounts and savings deposits			0
Time deposits			0
Total deposits			0
Securities sold under agreement to repurchase			0
FHLB borrowings			47
Accrued interest and other liabilities			1,530
Total liabilities assumed			1,577
Total equity assumed			(54,556)
Total liabilities and equity assumed			(52,979)
Citizens National Bank | Fair Value Adjustments | Core deposit intangible
Business Acquisition [Line Items]
Intangibles			5,075
Citizens National Bank | Fair Value Adjustments | Other intangibles
Business Acquisition [Line Items]
Intangibles			$ 591